Can anyone please help me with the following: “con i trofei da lui venti a calcetto”. I think that the correct translation is - “with his trophies he won at soccer” . But, this sentence is very confusing to me. Is the preposition “da” included to show the purpose of the trophies? Is “venti” the inflected form of the adjective “vinto” that means “won”?

I would translate it as With the trophies he won at five-a-side football. A literal translation into English is With the trophies by him won at five-a-side football. In other words, da means by so da lui is by him.Vinti is the plural of vinto, the past participle of vincere, to win and functioning as an adjective here (technically, I think, part of the adjectival phrase vinti da lui but I am no grammarian). It has to be in the plural because it is modifying the plural noun trofei.

I was wondering if you would agree that this sentence is in the passive voice and that the preposition “da” (by) is being used to introduce the performer of the action (him).

Here is the complete sentence:

Alle pareti ci sono due poster del suo complesso preferito e una mensola con i trofei da lui vinti a calcetto.
On the walls there are two posters of his favorite band and a shelf with the trophies he won at five-a-side football.

I'm not a grammar expert Rich but I would say it is an adjectival clause in the passive voice. Expressed actively it would be che lui ha vinto a calcetto. In other words, you are correct except that it is a clause, not a sentence. In the complete sentence, the verb in the main clause is, of course, essere.